220
Chapter 7
(a)
(b)
(c)
Cochlea
Scala
vestibuli
Cochlear
duct
Organ of Corti
Scala
tympani
Tectorial membrane
Stereocilia
Hair cells
Nerve fibers
Blood vessel
Basilar
membrane
Organ of Corti
Cochlear branch of
vestibulocochlear
nerve
duct that begins at the oval window; and the
scala tympani,
which is below the cochlear duct and ends in a second mem-
brane-covered opening to the middle ear, the
round window.
The scala vestibuli and scala tympani meet at the end of the
cochlear duct at the
helicotrema
(see Figure 7–35).
Sound waves in the ear canal cause in-and-out move-
ment of the tympanic membrane, which moves the chain of
middle-ear bones against the membrane covering the oval
window, causing it to bow into the scala vestibuli and back
out (
Figure 7–36
). This movement creates waves of pressure
in the scala vestibuli. The wall of the scala vestibuli is largely
bone, and there are only two paths by which the pressure
waves can dissipate. One path is to the helicotrema, where the
waves pass around the end of the cochlear duct into the scala
tympani. However, most of the pressure is transmitted from
the scala vestibuli across the cochlear duct. Pressure changes
in the scala tympani are relieved by movements of the mem-
brane within the round window.
The side of the cochlear duct nearest to the scala tym-
pani is formed by the
basilar membrane
(
Figure 7–37
),
upon which sits the
organ of Corti,
which contains the ear’s
Figure 7–37
Cross section of the membranes and compartments of the inner
ear with detailed view of the hair cells and other structures on
the basilar membrane. Views (a), (b), and (c) show increasing
magnifi cation.
Redrawn from Rasmussen.
Middle
ear bones
move
External auditory canal
Tympanic
membrane
deflects
Membrane
in round
window
moves
Basilar
membrane
moves
Membrane in
oval window
moves
Cochlear duct
(2)
(1)
(5)
(4)
(3)
Figure 7–36
Transmission of sound vibrations through the middle and inner ear.
Sound waves coming through the external auditory canal move the
tympanic membrane (1), which sets off a sequence of events, moving
the bones of the middle ear (2), the membrane in the oval window
(3), the basilar membrane (4), and the round window membrane (5).
Redrawn from Davis and Silverman.
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